The invention relates to a circuit substrate whose thermal radiation property is improved by a mixture of resin and inorganic filler. In particular, it relates to a high thermal radiation printed wiring board made of resin (thermally conductive substrate) for mounting electronic power devices.
Recently, as high performance and miniaturization of the electronic apparatus have been required, high density and high performance semiconductors have been sought. Consequently, circuit substrates for mounting thereof have also been required to be small and of high density. As a result, it is important to design circuit substrates taking the thermal radiation property into consideration. A well known technique for improving the thermal radiation property of circuit substrates, while using a printed circuit board made of glass-epoxy resin, is to use, a metal base substrate having a metal, for example, aluminum etc. and form a circuit pattern on one face or both faces of this metal substrate with an insulating layer interposed in between the circuit pattern and the metal substrate. Moreover, when higher thermal conductivity is required, the metal base substrate is made of a copper plate, which is directly bonded to a ceramic substrate made of, for example, alumina or aluminum nitride. For an application requiring relatively small electric power, a metal base substrate is generally used. In this case, however, in order to improve the thermal conduction, the insulating layer must be thin. Therefore, as for the substrate of thin insulating layer, break down voltage is low, and the influence by the noise, too, is big.
It is difficult for the metal base substrate and ceramic substrate to satisfy both performance and cost requirements. Recently, an injection molded thermally conductive module has been suggested, where a thermoplastic resin composition containing inorganic filler is integrated with the lead frame of an electrode. This injection molded thermally conductive module has excellent mechanical strength in comparion with a ceramic substrate. However, due to the high viscosity of the thermoplastic resin, it is difficult to injection mold such a module with a high filler content, and so the thermal radiation property of module is poor. In particular, at the time of melting the thermoplastic resin at high temperature and kneading with filler, if there is too much filler, the melting viscosity is rapidly increased in a point that not only kneading but also injection molding is made impossible. Moreover, the filler serves as an abrasives to abrade the metallic mold, and, thus, reduces the life of the mold. Consequently, the content of the filler is limited, so that only lower thermal conductivity can be obtained as compared with the thermal conductivity of the ceramic substrate.
The object of the present invention is to overcome the above mentioned problems and to provide a sheet for a thermally conductive substrate in which an inorganic filler can be filled in a resin at a high filler loading to form a thermally conductive module by a simple method, having (a) approximately the same coefficient of the thermal expansion in the plane direction of the substrate as that of a semiconductor, and (b) excellent thermal radiation property; a method for manufacturing the above mentioned sheet for a thermally conductive substrate; a thermally conductive substrate using the above mentioned sheet; and a method for manufacturing this thermally conductive substrate.
In order to attain the objects, the sheet for the thermally conductive substrate of the present invention is a sheet mixture comprising 70 to 95 weight parts of inorganic filler and 5 to 30 weight parts of resin composition comprising at least thermosetting resin, hardener and hardening accelerator. This sheet mixture has a good flexibility in the half hardened state or partially hardened state. (Hereinafter, xe2x80x9cB stagexe2x80x9d will be used for the half hardened state or partially hardened state.) This sheet mixture of the thermally conductive substrate can be molded and processed into a predetermined shape due to the flexibility of the sheet. On complete hardening of the resin composition, the substrate can be made rigid with excellent mechanical strength.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the half hardened state or partially hardened state has a viscosity in the range of 102 to 105 (Paxc2x7s). By such a preferred embodiment, excellent flexibility and processing property can be provided, so that the sheet can be molded and processed into the predetermined shape. More preferably, the half hardened state or partially hardened state has a viscosity in the range of 103 to 104 (Paxc2x7s). The viscosity of the sheet herein is measured by the following method: the apparatus used for measuring the elasto-viscosity was a xe2x80x9ccone and platexe2x80x9d type dynamic measurement apparatus, MR-500, the product of Rhelogy Co., Ltd.; the sheet was processed into the predetermined size and sandwiched between the cone and plate having a diameter of 17.97 mm and cone angle of 1.15 deg.; sinusoidal oscillation was given to the sample in the twisting direction; and the difference in the phases of torque which was generated by the sinusoidal oscillation was calculated. Thus, the viscosity was measured. In the evaluation of the elasto-viscosity of the sheet of the present invention, the sinusoidal oscillation was a sine wave with a frequency of 1 Hz, the strain was 0.1 deg., the load was 500 g and the temperature was 25xc2x0 C.
It is preferable in the sheet for the thermally conductive substrate of the present invention that 0.1 to 2 weight parts of solvent having a boiling point of not less than 150xc2x0 C. is further added to 100 weight parts of total weight of inorganic filler and thermosetting resin composition. By this preferred embodiment, excellent flexibility and processing property can be provided.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the solvent having a boiling point of not less than 150xc2x0 C. is at least one solvent selected from the group consisting of ethyl carbitol, butyl carbitol and butyl carbitol acetate. By this preferred embodiment, the processing of the sheet material is easy, flexibility can be provided to the thermosetting resin at room temperature, and the viscosity of the sheet material for molding and processing can easily be controlled.
It is preferable in the thermosetting resin composition in the sheet for the thermally conductive substrate of the present invention to comprise:
1) 0 to 45 weight parts of a first resin that is solid at room temperature,
2) 5 to 50 weight parts of a second resin that is liquid at room temperature,
3) 4.9 to 45 weight parts of the hardener, and
4) 0.1 to 5 weight parts of the hardening accelerator
when the thermosetting resin composition is 100 weight parts. By such a preferred embodiment, excellent flexibility and processing property can be provided.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the main component of the thermosetting resin that is solid at room temperature is one or more components selected from the group consisting of bisphenol A epoxy resin, bisphenol F epoxy resin and liquid phenol resin. By this preferred embodiment, the xe2x80x9cB stagexe2x80x9d resin has a long shelf life and the hardened resin has excellent electrical insulating property and mechanical strength.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the main component of the thermosetting resin composition is at least one resin selected from the group consisting of epoxy resin, phenol resin and cyanate resin.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the thermosetting resin composition comprises brominated multifunctional epoxy resin as a main component, bisphenol A novolak resin as a hardener, and imidazole as a hardening accelerator. By such a preferred embodiment, the substrate can be made excellent in flame retardant property, electric insulating property and mechanical strength.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the brominated multifunctional epoxy resin be in the range of 60 to 80 weight parts; bisphenol A novolak resin as a hardener be in the range of 18 to 39.9 weight parts, and imidazole as a hardening accelerator be in the range of 0.1 to 2 weight parts.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the inorganic filler is at least one kind of filler selected from the group consisting of Al2O3, MgO, BN and AlN, because these fillers are excellent in thermal conductivity.
It is preferable in the sheet for the thermally conductive substrate of the present invention that at least one additives is selected from the group consisting of coupling agent, dispersing agent, coloring agent and tack free agent is further added to the sheet for a thermally conductive substrate.
Next, the thermally conductive substrate of the present invention is characterized in that when the thermosetting resin component of the thermally conductive substrate sheet is hardened, the coefficient of thermal expansion is in the range of 8 to 20 ppm /xc2x0 C. and the thermal conductivity is in the range of 1 to 10 W/mK. In the thermally conductive substrate, thermal deformation or the like is not generated and the coefficient of thermal expansion approximates that of a semiconductor.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the flexural strength of the thermally conductive substrate is not less than 10 Kgf/mm2. If the flexural strength is within the above mentioned range, practical mechanical strength can be obtained. The flexural strength herein is measured according to JIS R-1601 (the testing method of bending strength of fine ceramics) in the following manner: test sample is cut in a predetermined size; the test sample is placed on two supporting points which are located at certain distance; load is applied to the middle point of the test sample between two supporting points, the maximum bending stress when the test sample breaks is measured and this value is defined as flexural strength. This value is also called the three-point bending strength.
The dimensions of the test sample are as follows:
Whole Length (Lr): 36 mm
Width (w): 4.0xc2x10.1 mm
Thickness (t): 3.0xc2x10.1 mm
The bending strength is calculated by the following equation:
"sgr"=3PL/2 wt2 
wherein "sgr" denotes the three-point bending strength (kgf/mm2), P denotes the maximum load when the test piece is broken, L denotes the distance between lower supporting points (mm), w denotes the width of the test piece (mm) and t denotes the thickness of the test piece (mm).
It is preferable in the sheet for the thermally conductive substrate of the present invention that the flexural strength is in the range of 10 to 20 Kgf/mm2.
It is preferable in the sheet for the thermally conductive substrate of the present invention that a lead frame is further integrated to the thermally conductive substrate, and the thermally conductive substrate is filled to the surface of the lead frame. By such a preferred embodiment, electronic parts can easily be attached to the lead frame and thermal resistance for connecting thermal radiation can be inhibited. Moreover, soldering terminals for connecting a removable electrode are not required. Instead, the lead frame can be connected directly to an outside signal source, which may be an electrode for taking current. Thus, reliability by such a preferred embodiment is excellent.
It is preferable in the sheet for the thermally conductive substrate of the present invention that a metal substrate for thermal radiation is further formed on the face opposite to the face to which the lead frame is adhered to the thermally conductive substrate. By such a preferred embodiment, thermal resistance can be further decreased and the mechanical strength is improved.
It is preferable in the sheet for the thermally conductive substrate of the present invention that a printed circuit board having two or more wiring layers be integrated into a part of the face of the thermally conductive substrate to which the lead frame is adhered, the thermally conductive substrate be filled to the surface of the lead frame, and the printed circuit board comprises two or more wiring layers. By such a preferred embodiment, the control circuit for overcurrent protection or temperature compensation can be integrated into the substrate. Thus, miniaturization and high density of the apparatus can be realized.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the thermally conductive substrate has a through hole. The through hole is filled with conductive resin composition or is plated with copper, and a metallic foil wiring pattern is formed and integrated on both sides of the substrate. By such a preferred embodiment, double-sided wiring substrate which is excellent in thermal radiation can be obtained.
It is preferable in the sheet for the thermally conductive substrate of the present invention that a plurality of the thermally conductive substrates are layered and each thermally conductive substrate has a through hole. The through hole is filled with conductive resin composition and an internal wiring pattern is composed of conductive resin composition. In addition, a metallic foil wiring pattern is formed and integrated on both sides of the substrate. By such a preferred embodiment, conductivity between layers of the thermally conductive substrate is excellent and internal wiring pattern can be formed. Furthermore, excellent thermal conductivity can be provided.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the metallic foil is a copper foil having a thickness of 12 to 200 xcexcm and having faces at least one surface of which is a rough surface.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the conductive resin composition comprises 70 to 95 weight parts of at least one metallic powder selected from the group consisting of silver, copper and nickel: and 5 to 30 weight parts of thermosetting resin and hardener.
It is preferable in the sheet for the thermally conductive substrate of the present invention that the inorganic filler has an average particle diameter of 0.1 to 100 xcexcm.
The first method of manufacturing the sheet for the thermally conductive substrate of the present invention comprises the steps of: forming a slurry mixture comprising 70 to 95 weight parts of an inorganic filler, 4.9 to 28 weight parts of a thermosetting resin composition and 0.1 to 2 weight parts of a solvent having a boiling point of not less than 150xc2x0 C. and solvent having a boiling point not more than 100xc2x0 C.; forming the slurry mixture into a film having a desired thickness; and drying the solvent having a boiling point of not more than 100xc2x0 C. of the film slurry.
The second method of manufacturing the sheet for the thermally conductive substrate of the present invention comprises the steps of: forming a slurry mixture comprising 70 to 95 weight parts of inorganic filler, 5 to 30 weight parts of thermosetting resin mixture comprising a solid of thermosetting resin that is solid at room temperature and a liquid thermosetting resin that is liquid at room temperature and solvent having a boiling point not more than 100xc2x0 C.; forming the slurry mixture into a film having a desired thickness; and drying only the solvent having a boiling point of not more than 100xc2x0 C. of the film slurry.
It is preferable in the second manufacturing method that the thermosetting resin mixture in the sheet for thermally conductive substrate made according to the second method, comprises:
1) 0 to 45 weight parts of resin that is solid at room temperature,
2) 5 to 50 weight parts of resin that is liquid at room temperature,
3) 4.9 to 45 weight parts of hardener, and
4) 0.1 to 5 weight parts of hardening accelerator
when the total weight of the solid thermosetting resin and the liquid thermosetting resin 100 is weight parts.
It is further preferable in the second manufacturing method that the main component of the solid thermosetting resin is one or more components selected from the group consisting of bisphenol A epoxy resin, bisphenol F epoxy resin and liquid phenol resin.
It is preferable in the first and second manufacturing methods that the thermosetting resin mixture comprises a brominated multifunctional epoxy resin as a main component, a bisphenol A novolak resin as a hardener, and an imidazole as a hardening accelerator.
It is preferable in the first and second manufacturing methods that the sheet for a thermally conductive substrate comprises a brominated multifunctional epoxy resin in the range of 60 to 80 weight parts; a bisphenol A novolak resin as a hardener in the range of 18 to 39.9 weight parts, and an imidazole as a hardening accelerator in the range of 0.1 to 2 weight parts.
It is preferable in the first manufacturing method that the solvent having a boiling point of not less than 150xc2x0 C. is at least one solvent selected from the group consisting of ethyl carbitol, butyl carbitol and butyl carbitol acetate.
It is preferable in the first and second manufacturing methods that the solvent having a boiling point of not more than 100xc2x0 C. is one solvent selected from the group consisting of methyl ethyl ketone, isopropanol and toluene.
It is preferable in the first and second manufacturing methods that an additive selected from the group consisting of coupling agent, dispersing agent, coloring agent and tack free agent is further added to the sheet for a thermally conductive substrate.
It is preferable in the first and second manufacturing methods that the film forming method is at least one method selected from the group consisting of doctor blade method, coater method, and injection molding method.
The third method for manufacturing the thermally conductive substrate of the present invention comprises the steps of: piling up a lead frame on a face of the sheet for the thermally conductive substrate manufactured by the first manufacturing method; molding the sheet at a temperature below the hardening temperature of the thermosetting resin composition and at a pressure in the range of 10 to 200 Kg/cm2; filling the sheet and integrating to the surface of the lead frame; and hardening the thermosetting resin by thermal pressing at the pressure in the range of 0 to 200 Kg/cm2.
It is preferable in the third manufacturing method that a metal substrate for thermal radiation is further formed on the face opposite to the face to which the lead frame is adhered to the thermally conductive substrate.
Moreover, the third method for manufacturing the thermally conductive substrate of the present invention comprises the steps of: placing the lead frame and a printed circuit board having two or more wiring layers on the sheet for the thermally conductive substrate manufactured by the first manufacturing method in a way in which the lead frame and the printed circuit board are not overlapped; molding the sheet at the temperature below the hardening temperature of the thermosetting resin composition and at the pressure in the range of 10 to 200 Kg/cm2; filling the sheet and integrating to the surface of the lead frame and the printed circuit board having two or more wiring layers; and hardening the thermosetting resin by thermal pressing at the pressure of 0 to 200 Kg/cm2.
Moreover, the third method for manufacturing the thermally conductive substrate of the present invention comprises a series of steps of: processing through holes on the sheet for the thermally conductive substrate manufactured by the first manufacturing method; filling a conductive resin composition into the through holes; piling up the metallic foil on both sides of the sheet into which the conductive resin composition is filled in the through holes; hardening the thermosetting resin of the sheet by thermal pressing at the pressure of 10 to 200 Kg/cm2; and forming wiring pattern by processing the metallic foil.
Moreover, the method for manufacturing the thermally conductive substrate of the present invention comprises the steps of: piling up a metallic foil on the both sides of the sheet for the thermally conductive substrate manufactured by the first manufacturing method; hardening the thermosetting resin of the sheet of thermally conductive substrate by thermal pressing at the pressure of 10 to 200 Kg/cm2; processing through holes on the hardened the thermally conductive sheet; conducting a copper plating on the entire surface of the sheet on which through holes are processed; and forming a wiring pattern by processing the metallic foil and the copper plating layer.
Moreover, the third method for manufacturing the thermally conductive substrate of the present invention comprises the steps of: preparing a desired number of thermally conductive substrates by the first manufacturing method; processing through holes at desired locations on each of the sheets; filling a conductive resin composition into the through holes; forming a wiring pattern on one surface of the filled sheet by using the conductive resin composition; piling up each of the sheet having the wiring pattern in a way in which the surface having the wiring pattern is adjusted to face upward and the sheet on which only the conductive resin composition is filled to the through hole is adjusted to be the top face to form a pile; piling up metallic foil on both sides of the pile; hardening the thermosetting resin of the sheet for the thermally conductive substrate by thermal pressing at the pressure of 10 to 200 Kg/cm2; and forming a wiring pattern by processing the metallic foil.
It is in the third manufacturing method that the through holes are processed by the method selected from the group consisting of laser beam process, drilling process and punching process.
It is in the third manufacturing method that the metallic foil is a copper foil having a thickness of 12 to 200 xcexcm and having faces at least one surface of which is a rough surface.
It is in the third manufacturing method that the conductive resin composition comprises 70 to 95 weight parts of at least one metallic powder selected from the group consisting of silver, copper and nickel; and 5 to 30 weight parts of thermosetting resin and hardener.
It is in this third manufacturing method that the temperature for the thermal pressing is in the range of 170 to 260xc2x0 C.
As mentioned above, according to the present invention, high thermal radiation printed circuit wiring board for mounting electronic power devices can be made of the thermally conductive substrate by shaping and hardening the thermally conductive sheet into a desired shape. Shaping is possible due to the flexibility of the thermally conductive substrate sheet, hardening makes the thermally conductive substrate rigid.
Moreover, according to the present invention, thermally conductive substrate can be manufactured efficiently and reasonably.
The first embodiment of the present invention basically relates to a thermally conductive sheet having flexibility, where an inorganic filler is added into a thermosetting resin in the not-hardened state at high density; the coefficient of thermal expansion in the plane direction is approximately the same as that of Si semiconductor; and high thermal conductivity is provided. In the thermally conductive sheet of the present invention, a high boiling point solvent is added into the thermosetting resin composition, or a thermosetting resin mixture containing a solid resin that is solid at room temperature and a liquid thermosetting resin that is liquid at room temperature, and films are formed by using a low boiling point solvent for mixing with inorganic filler. Consequently, in the thermally conductive sheet of the present invention, inorganic filler can be added at a high filler loading. Furthermore, the flexibility of the thermosetting resin of the thermally conductive sheet is manufactured in the not-hardened state, and, thus, molding the thermally conductive sheet into a desired shape at a low temperature and at a low pressure is possible. In addition, the thermally conductive substrate can be made rigid by hardening the thermosetting resin by thermal pressing. Also, a thermally conductive substrate on which a semiconductor can be simply and directly mounted can be obtained by the use of this thermally conductive sheet which is flexible.
The second embodiment of the present invention relates to a thermally conductive substrate on which a semiconductor having thermal radiation property can directly be mounted by using the thermally conductive sheet; piling up a lead frame; and hardening the thermally conductive sheet by means of thermal pressing to integrate with the lead frame.
Moreover, the third embodiment of the present invention relates to a doubled-sided thermally conductive substrate having high thermal conductivity, which permits electrical conductivity on both sides by forming through holes on the thermally conductive sheet, filling the thorough holes with the conductive resin composition and forming metallic foil patterns on both sides of the sheet.
Moreover, the fourth embodiment of the present invention relates to a high thermally conductive double-sided substrate which permits electric conductivity by copper plating to the through holes of the third embodiment.
Moreover, the fifth embodiment of the present invention relates to a thermally conductive substrate (a multi-layered substrate) having a multi-layered circuit structure in which a plurality of the thermally conductive sheets are used, the through holes to which conductive resin composition is filled are formed, wiring pattern is formed on one side of the thermally conductive sheet, and a plurality of the thermally conductive sheets are piled up.